Most of the calculation is straightforward, but there's one key variable: the kind of person who will do the screening. You can't just use random people off the street, or starving artists, or bored software engineers, because the crucial question they must answer is: does too much of this video infringe on somebody's copyright? Only one class of person is qualified to answer that, and hence to take on this job: judges. Or, more specifically:

horribly underpaid judges who happen to be extremely efficient at determining the copyright status of each video they watch and choose, of the little free will they have, to consider all video uploaded.

Using the fact that the average pay for a judge in Silicon Valley is apparently $177,454, and that based on the volume of uploads and number of hours in a working day, a mere 199,584 judges would be required as screeners, this gives us the final figure for the cost of checking properly those 72 hours per minute:

Absurd as this calculation may be, it does reveal the key problem with unthinking calls for YouTube videos to be pre-screened for possible infringement: only suitably-qualified individuals can do that, and eventually you run out of them. In other words, attempts to police rigorously online materials are doomed to fail by the nature of the copyright system itself. Basically, copyright does not scale.